>>728
ほんとに開いてサイズ変更して保存するだけですが。

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import javax.swing.*;
public class Q728 {
public static void main(String[] args) throws Exception {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
JFileChooser fileChooser = new JFileChooser() {
public boolean accept(File f) { return f.isDirectory() || f.getName().endsWith(".jpg"); }
};

int stat = fileChooser.showOpenDialog(null);
if (stat == JFileChooser.CANCEL_OPTION || stat == JFileChooser.ERROR_OPTION) return;
BufferedImage src = ImageIO.read(fileChooser.getSelectedFile());
double d = Double.parseDouble(JOptionPane.showInputDialog("倍率を入力してください。 (例: 1.5)"));
int w = (int)(src.getWidth() * d);
int h = (int)(src.getHeight() * d);
if (w <= 0 || w > 5000 || h <= 0 || h > 5000) return;
BufferedImage dst = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
Graphics2D g = (Graphics2D) dst.getGraphics();
g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
g.drawImage(src, 0, 0, w, h, null);
stat = fileChooser.showSaveDialog(null);
if (stat == JFileChooser.CANCEL_OPTION || stat == JFileChooser.ERROR_OPTION) return;
ImageIO.write(dst, "jpg", fileChooser.getSelectedFile());
}
}